WO2025040677A1

WO2025040677A1 - Detection of zooplankton within a volume of water

Info

Publication number: WO2025040677A1
Application number: PCT/EP2024/073345
Authority: WO
Inventors: Knud Poulsen; Frederik TAARNHØJ; Isabel Eline HENDRIKS; Zandra Maria Skandrup SIGVARDT
Original assignee: Fp Aquaculture & Ocean Health Aps
Current assignee: Fp Aquaculture & Ocean Health Aps
Priority date: 2023-08-21
Filing date: 2024-08-20
Publication date: 2025-02-27
Anticipated expiration: 2026-02-21

Abstract

An apparatus for detecting freely swimming zooplankton in a body of water, the apparatus comprising one or more light sources for emitting visible light and the apparatus being configured to direct a first light portion of the emitted light into the body of water to attract the freely swimming zooplankton towards a detection zone and to direct a second light portion of the emitted light onto the detection zone for illuminating zooplankton within the detection zone, the apparatus further comprising an imaging device configured to capture one or more images of one or more zooplankton present within the detection zone.

Description

DETECTION OF ZOOPLANKTON WITHIN A VOLUME OF WATER

TECHNICAL FIELD

The present disclosure relates to the detection and/or classification of zooplankton, in particular of larval stages of ectoparasites such as salmon lice or "skottelus," within a volume of water.

BACKGROUND

Salmon lice (L. salmonis) and "skottelus" (C. elongatus) are ectoparasites, which host primarily on salmon and have a significant economic impact on the salmon farming industry due to salmon lice control, treatment and reduced health and growth of the infected fish. In addition, infestations of salmon lice and/or skottelus in farms may spread and affect populations of wild salmon and trout, thus additionally impacting wildlife and ocean health.

Today, treatment and monitoring are primarily targeted towards the parasitic life stages of the lice, i.e. the stages during which the louse is attached to a host.

In EP 2 962 556 monitoring of ectoparasites on fish is done by illuminating a fish with illumination radiation capable of inducing fluorescence in ectoparasites and detecting any induced fluorescence using one or more detectors. Any ectoparasites detected using the method and system of EP 2 962 556 are ones that have attached themselves to the fish and are in their parasitic life stage.

More knowledge about the abundance and distribution of the planktonic, larval stages in which the lice are freely swimming could improve preventive treatment methods, treatment planning and mathematical life cycle and spread models. However, such knowledge is currently limited to what can be gained from manual sampling using plankton nets or water pumps, which is costly, time-consuming and susceptible to local variations. As such, a method for real-time, automated detection of larval salmon lice and/or skottelus etc. is a desired alternative.

Plankton, which are organisms living in bodies of water, are divided into zooplankton (animals) and phytoplankton (plants). A characteristic of plankton is that it cannot swim well enough to move against tides and currents, and while some are able to change direction or rise and sink, they primarily drift in water. Freely swimming is therefore to be understood in the sense that the zooplankton is not attached to anything, e.g. a host.

It is generally desirable to provide a method and an apparatus for detecting zooplankton, in particular salmon lice larvae (SLL) and/or skottelus larvae, within a volume of water.

WO 2020/239833 discloses a method for detecting freely swimming non- phytoplankton-eating zooplankton. This prior art method comprises: illuminating a volume of water with a first illumination radiation comprising a first illumination wavelength, the first illumination radiation being capable of inducing autofluorescence of chlorophyll; detecting radiation received from the volume of water; determining whether zooplankton is present within the volume of water based on the detected radiation; measuring an amount of autofluorescence from chlorophyll from a zooplankton present within the volume of water; identifying whether the zooplankton determined to be present within the volume of water is a non-phytoplankton-eating zooplankton based on the measured amount of autofluorescence from chlorophyll.

It remains desirable to provide alternative solutions that can be implemented and produced at relatively low cost and/or in large numbers, are reliable, robust, safe in use and/or that can serve as an alternative to existing solutions. In particular, it remains desirable to provide a method and apparatus that allows detection of zooplankton even when the zooplankton is only present at relatively low concentrations and in the presence of other organisms. It is desirable to provide a method having a good signal-to-noise ratio.

SUMMARY

In one aspect, an apparatus for detecting freely swimming zooplankton in a body of water is disclosed. The apparatus comprises one or more light sources for emitting visible light. The apparatus is configured to direct a first light portion of the emitted light into the body of water so as to attract the freely swimming zooplankton towards a detection zone. The apparatus is further configured to direct a second light portion of the emitted light onto the detection zone for illuminating zooplankton within the detection zone. The apparatus further comprises an imaging device configured to capture one or more images of the detection zone and, in particular, of one or more zooplankton present within the detection zone.

By attracting the zooplankton to be imaged towards the detection zone, the apparatus is capable of reliably detecting even small concentrations of zooplankton within the body of water.

In some embodiments, the apparatus further comprises an image processing unit configured to process the captured one or more images to recognize and/or classify one or more zooplankton in the one or more captured images.

Accordingly, embodiments of the apparatus are useful for the detection and/or classification of zooplankton in a body of water. Various embodiments of the apparatus may be used for in situ measurements in a waterbody, such as an ocean, lake, river or the like, e.g. in or near a fish farm. The waterbody may be a natural waterbody or a human-made waterbody, e.g. an artificial pond. Some embodiments may be used for detecting and/or classifying zooplankton in an artificial water container, such as in a tank, tube, pipe etc.

Embodiments of the apparatus described herein may be used for detecting and/or classifying various types, in particular various species, of zooplankton, e.g. feeding organisms such as copepods. Examples of zooplankton include larval stages of certain ectoparasites, such as salmon lice, (L. salmonis) and/or "skottelus" (C. elongatus) and/or the like. However, various embodiments of the apparatus disclosed herein may detect and/or classify other types of zooplankton.

Embodiments of the apparatus disclosed herein may have various uses, including but not limited to:

- As a tool to detect and/or classify and/or quantify the amount of zooplankton, in particular of one or more predetermined types/species of zooplankton, within a body of water, e.g. within or near a fish farm.

- As a tool for evaluation of ocean health, e.g. by detecting which species are present where, by assessing biodiversity, and/or the like. - As a tool to detect and/or quantify infestation or risk of imminent infestation of a body of water with ectoparasites or other undesirable organisms, e.g. by detecting and/or classifying and/or quantifying the amount of larval stages of the ectoparasites.

- As a tool for RAS systems.

- As a tool for the automated monitoring of e.g., biological organisms in tubes or the like.

It has turned out that detection and/or classification of zooplankton can efficiently and reliably be done by image processing of images acquired by embodiments of the apparatus.

Embodiments of the apparatus may easily be produced and at relatively low cost and they are robust, and easy to install.

Embodiments of the apparatus provide direct, reliable measurements.

Embodiments of the apparatus are safe in use, in particular eye-safe.

The apparatus may comprise a submersible housing configured to accommodate the one or more light sources and the imaging device and to be submersed into the body of water. Accordingly, in some embodiments, the apparatus is or comprises a submersible unit configured to be submersed into the body of water and to operate while being submersed in the body of water.

The imaging device may comprise a digital camera. The imaging device may comprise a microscope or another suitable optical system for imaging zooplankton moving about the detection zone.

The image processing unit may also be accommodated within the submersible housing. Alternatively, the image processing unit, or a part thereof, may be located external to the submersible housing, e.g. above-water, e.g. on shore, on a floating vessel or the like. The image processing unit may comprise a suitably programmed data processing device, e.g. a suitable programmed computer, a GPU, and/or the like. The apparatus may further comprise, preferably accommodated in said submersible housing, a communications interface for communicating the captured images and/or the image processing results to an external data processing system, external to the submersible housing, and/or to a control system for controlling one or more parasite countermeasures, which may also be accommodated external to the submersible housing. The communications interface may communicate the data via a wired or a wireless connection.

The apparatus may comprise a power source, e.g. a battery, accommodated within the submersible housing and/or be connectable to an external power supply, e.g. via a suitable power cable.

The detection zone may partly or completely be defined by a transparent surface, e.g. a planar surface, to which the first light portion attracts the zooplankton. The detection zone may thus be a quasi-two-dimensional area, or a relatively thin layer of water directly adjacent to the transparent surface, on the outer side - relative to the apparatus - of the transparent surface. The imaging device may thus image the detection zone through the transparent surface. The thickness of the detection zone in a direction away from the transparent surface may be defined by the focal depth of the imaging device. In some embodiments, the thickness of the detection zone is less than 1 cm such as between 0.5 mm and 2 cm, e.g. between 1 mm and 1 cm. However, in other embodiments, the detection zone may be a volume having a larger depth such as up to 10 cm.

The transparent surface defining the detection zone, e.g. the surface portion imaged by the imaging device may be 1 cm² or more, such as at least 3 cm², such as at least 5 cm², such as at least 10 cm².

Imaging a relatively large detection zone increases the sensitivity of the system.

Generally, various embodiments of the apparatus attract the zooplankton to be detected and/or classified (e.g SLL and/or skottelus larvae and/or the like), with light to increase the number of the zooplankton to be detected and/or classified in the detection zone / in the frame for photographing. In some embodiments, the apparatus is configured to emit the first and second light portions via a light-emitting surface area of the submersible housing. The first light portion may be emitted as attraction light into the body of water for attracting the zooplankton towards the detection zone, while the second light portion may be directed towards the detection zone. The emitted light may be emitted as a light beam, such as a collimated beam, a diverging beam or as a diffused beam, or otherwise. In particular, the attraction light may be emitted as an attraction light beam, , such as a collimated beam, a diverging beam or as a diffused beam, or otherwise.

The apparatus may e.g. include multiple light sources, e.g. a plurality of LEDs. A first set of the light sources may be controllable to selectively emit the first light portion and a second set of the multiple light sources may be controllable to emit the second light portion, either concurrently with the first light portion or such that the first and second light portions are emitted alternatingly. The first set of light sources may include a subset, or the entire set, of light sources. The second set of light sources may be a subset of the first set or a different subset of the entire set of light sources. The first and second subsets of light sources may all be light sources of the same type. For example, the apparatus may include an array of LEDs of the same type, which may be used to emit the first light portion while a subset of the LEDs may be used to emit the second light portion. It will be appreciated that other types of light sources may be used.

In some embodiments, the apparatus may be configured to emit the first and second light portions concurrently, e.g. as an output beam, where a first beam portion of the output beam provides the first light portion and a second beam portion of the output beam provides the second light portion. Hence, the apparatus may be implemented with a single light source, or a set of light sources that are controlled to emit light concurrently with each other.

To this end, the apparatus may comprise a reflector, in particular a passive reflector, configured to receive the second light portion and to redirect the second light portion towards the detection zone. The reflector may e.g. be a diffuse reflector. In one embodiment, the reflector provides a bright field illumination of the detection zone, i.e. such that direct (i.e. unscattered by the objects to be imaged) light from the reflector reaches the imaging device. The reflector may e.g. be positioned in the beam path of the emitted output beam and configured to reflect a portion of the output beam. To this end, the reflector may be semitransparent and semi-reflective and/or the reflector may be shaped, sized and or positioned such that it does not block the entire cross section of the output beam, i.e. such that a portion of the output beam can illuminate the body of water beyond the position of the reflector, i.e. at a distance from the detection zone larger than the distance between the reflector and the detection zone.

In another embodiment, the reflector may be configured to provide a dark field illumination of the detection zone, i.e. such that no direct light from the reflector reaches the imaging device. To this end, the reflector may be an annular reflector.

In some embodiments, the apparatus, in particular the submersible housing, comprises a transparent surface. For example the submersible housing, or a part thereof, may be formed by a transparent material such as plexiglas or the like. A first surface portion of the transparent surface may define the light-emitting surface and a second surface portion of the transparent surface may define the detection zone, which in this case may be defined as a detection area or a thin detection layer directly adjacent the second surface portion. The imaging device may thus be configured to have a focal plane at or directly adjacent to the second surface portion of the transparent surface.

The imaging device may comprise a microscope including a digital camera for imaging the detection zone, e.g. the second surface portion described above, and of zooplankton moving across the detection zone. Preferably, the imaging device is configured to provide a high-resolution image of the detection zone.

Preferably, the first and second surface portions of the transparent surface are adjacent, in particular directly adjacent, to each other. The inventors have realised that a particularly high portion of the zooplankton attracted towards the first surface portion enters the detection zone defined by the second surface portion, when the first surface portion surrounds the second surface portion, e.g. as an annular first surface portion. The reflector may be a passive reflector, which may be attached to the outside of the submersible housing via one or more arms or a frame or another suitable mounting structure. Preferably, the mounting structure is configured such that it allows a major portion of the first light portion emitted from light-emitting surface to illuminate the body of water beyond the mounting structure.

The image processing device may implement any suitable image processing and object recognition method, such as a machine learning process trained to recognize the zooplankton to be detected. Suitable machine-learning methods include deep learning methods and/or a YOLO process. Accordingly, a fast recognition of multiple zooplankton is possible and, thus, a determination of the amount of, or level of infestation with, a given species of zooplankton, e.g. of salmon lice larvae and/or skottelus larvae. For example, embodiments of the apparatus may be used to acquire large amounts of images that may serve as training and/or validation data for training and/or validation of a suitable machine-learning process, e.g. by supervised learning based on manually classified images, or otherwise.

Preferably, the wavelength range of the emitted light is selected such that it includes wavelength components by which the zooplankton is attracted. When the zooplankton to be detected are salmon lice larvae, wavelength components between 400 nm and 450 nm, e.g. between 410 nm and 430 nm, such as about 420 nm, have been found useful.

Embodiments of the apparatus can be manufactured at relatively low cost, are robust, yet provide reliable measurements of zooplankton infestation of a body of water in quasi real-time, e.g. of infestation by salmon lice larvae and/or skottelus larvae.

Generally, according to another aspect, disclosed herein are embodiments of an apparatus for detecting freely swimming zooplankton in a body of water, the apparatus comprising:

- a housing including a transparent member defining a transparent surface, - one or more light sources accommodated within the housing for emitting visible light and confined to direct light via the light-emitting portion of the transparent surface into the body of water,

- a reflector arranged outside the housing and configured to receive at least a portion of the light emitted by the one or more light sources via the lightemitting surface portion to redirect the received light onto a detection zone defined by a detection portion of the transparent surface,

- an imaging device accommodated within the housing and configured to capture one or more images of the detection zone.

According to yet another aspect, disclosed herein is a process for reducing infestation of sea animals, in particular of fish, such as salmon, more particularly of fish in fish farms, by ectoparasites, in particular salmon lice and/or skottelusand/or the like.

The process includes:

- detecting present infestation or a likelihood of imminent infestation of a body of water with ectoparasite larvae, preferably by an embodiment of the apparatus disclosed herein,

- controlling one or more ectoparasite countermeasures responsive to a detected infestation or a detected likelihood of imminent infestation.

The detection of a present infestation or a likelihood of imminent infestation of a body of water with ectoparasite larvae may include detecting a predetermined minimum concentration of said ectoparasite larvae, or a minimum increase of concentration over time or another suitable trigger criterion. The detection may occur at a detection location, which may e.g. be inside the fish pen or in a vicinity of the fish pen.

The control may be performed automatically or in a user-assisted manner.

Accordingly, countermeasures do not need to be performed unnecessarily, thereby saving energy, avoiding damage to the sea animals, avoiding resistance against the countermeasures and/or the like. On the other hand, as the control of the countermeasures is based on the detection of infestation with larval stages of the ectoparasites, the countermeasures may be initiated sufficiently early to substantially reduce or even prevent actual infestation with the ectoparasites.

Various countermeasures for preventing or at least reducing infestation of salmon with salmon lice are used or have been suggested. Examples include administering anti-parasitic substances, enclosing the sea animals in a housing, moving the sea animals to higher depths or other less infested regions, or even countermeasures targeting the parasites in their larval stage.

For example, timely treatment can be accomplished by triggering a feeding of the fish with feed mixed with for example the anti-parasitic agent "Slice" from Merck/MSD or similar anti-parasitic agent.

Another example of a timely countermeasure may include protecting the fish-pen with a skirt (which may be selectively raised or lowered) so as to block the salmon lice larvae from entering the pen. Typically, this skirt cannot be permanently in- place as it prevents oxygenated water from entering the pen, thereby suffocating the fish.

Another example of a timely countermeasure may include lowering the entire pen to deeper water to avoid the salmon lice larvae that are generally in the top 1-3 meters of the water column.

Another example of a timely countermeasure may include starting pumps at the sea floor that create a water curtain around the edge of the pen thereby creating a water-barrier around the pen and preventing salmon lice larvae from entering.

Here and in the following, the terms image processing unit and data processing unit are intended to comprise any circuit and/or device suitably adapted to perform the image and data processing functions, respectively, described herein. In particular, the above terms comprise a general- or special-purpose programmable microprocessor, such as a central processing unit (CPU) and/or graphics processing unit (GPU) of a computer or of another data processing system, a digital signal processor (DSP), an application specific integrated circuits (ASIC), a programmable logic arrays (PLA), a field programmable gate array (FPGA), a special purpose electronic circuit, etc., or a combination thereof. An image and/or data processing unit may be embodied as a single processor or as a distributed system including multiple processors.

The present disclosure also relates to a sea cage for accommodating salmon, the sea cage comprising an apparatus disclosed herein. The apparatus may be attached to the cage and/or otherwise mounted relative to the cage. The apparatus may be configured to detect zooplankton, in particular freely swimming salmon lice larvae and/or skottelus larvae and/or the like, inside the cage and/or in a proximity of the cage.

Additional features and advantages will be made apparent from the following detailed description of embodiments that proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of an apparatus for detecting freely swimming zooplankton.

FIG. 2 shows images of zooplankton species detected in a detection zone of an embodiment of the apparatus disclosed herein.

FIGs. 3A and 3B illustrate an apparatus for providing bright-field illumination and dark-field illumination, respectively.

DETAILED DESCRIPTION

The apparatus comprises a submersible housing 1, which may e.g. be made from steel or from another suitable material. The housing defines an enclosure for accommodating various components of the apparatus. The apparatus further comprises an imaging device 2, e.g. a microscope with a digital camera. The imaging device is accommodated within the housing 1.

The housing comprises a transparent member 4, e.g. a plexiglas plate or another suitable transparent housing component. The imaging device is configured such that its focal plane is located on or close to the outside surface of the transparent member 4. The field of view of the imaging device in the focal plane and the depth of field of the imaging device define a detection zone. A surface portion 4b of the transparent member may delimit the detection zone towards the housing.

The apparatus further comprises one or more light sources 2 accommodated within the housing 1. The one or more light sources emit light through a lightemitting portion 4a of the transparent member into the body of water into which the housing is submersed.

The apparatus further comprises a reflector 5 arranged outside of the enclosure defined by housing 1. The reflector 5 is configured such that it reflects a portion of the light that is emitted through surface portion 4a back towards the detection zone.

Another portion of the emitted light illuminates a portion of the body of water beyond the reflector so as to attract the zooplankton to be detected towards the transparent member 4. To this end, the light may be emitted via the lightemitting portion 4a as a collimated or divergent beam or diffused beam having a cross section that is larger than the cross section of reflector, i.e. such that only a part of the emitted light is blocked by the reflector 5.

The one or more light sources are configured such that the emitted light attracts the zooplankton to be detected and such that the reflected portion of the light, which is redirected towards the detection zone, provides a suitable illumination of zooplankton moving about the detection zone.

For example, when the zooplankton to be detected include the copepodid stage of the salmon lice larvae and/or skottelus larvae and/or other zooplankton to be detected, powerful blue and/or white LED's may be used as light sources to attract the zooplankton , and to provide suitable illumination for imaging with a high signal-to-noise ration.

Attracting the zooplankton to the surface portion 4b of the plexiglas barrier 4 has a significant benefit when it comes to focusing the camera/microscope on them, since the zooplankton is attracted towards a well-defined surface portion, i.e. at a well-defined distance to the camera. Accordingly, the imaging device only needs to have a limited focal depth. The use of a thin focal plane has the further benefit of suppressing imaging of detritus/ particles/ noise in the water.

A particular efficient attraction of the zooplankton towards the detection zone, without unnecessarily negatively effecting the imaging, may be achieved when the light-emitting surface portion 4a is adjacent the surface portion 4b that delimits the detection zone, in particular when the light-emitting surface 4a surrounds the surface portion 4b.

The apparatus further comprises a processing unit 7 configured to receive and process the image data from the imaging device to performs image processing and to implement a suitable object recognition process, e.g. a YOLO network. The processing unit may be or include a computer or another suitable processing unit. The processing unit may, as illustrated in FIG. 1, be accommodated within the submersible housing or it may at least in part be implemented by an external image and/or data processing system.

The apparatus may further comprise an interface unit 8 accommodated within the housing and configured to provide data communication with an external system, e.g. via a wired or wireless connection. Alternatively or additionally, e.g. in embodiments where a real-time detection is not required, the apparatus may include a data storage device for storing captured images such that the images may be read out at a later point in time after retrieval of the apparatus from its submersed measurement position.

FIG. 2 shows an image of zooplankton species detected in a detection zone of an embodiment of the apparatus disclosed herein. FIGs. 3A and 3B illustrate an apparatus for providing bright-field illumination and dark-field illumination, respectively.

The same light source or set of light sources may be used for attraction and for illumination while imaging in either bright field (see FIG. 3A) or dark field (see FIG. 3B)

For bright field illumination, the attracting and the illumination light may be emitted concurrently, and the illumination portion of the emitted light may be reflected back onto the detection zone by a basic diffuse reflector.

For dark-field illumination, some or all of the light sources used for emitting attraction light may be temporarily turned off during imaging so that only a subset of the light sources emits light. The reflector may e.g. be an annular reflector configured to redirect the light from the subset of light sources towards the detection zone such that the light reflected from the reflector does not directly reach the imaging device.

Alternatively or additionally, one or more acutely angled laser light-sheets may be used for dark-field illumination instead of an annular reflector. The laser lightsheets may be projected through the transparent member and exit into the water so as to illuminate the imaging plane near the transparrent barrier. The use of one or more laser light-sheets obviates the need for a protruding member that can easily catch sea-born detritus such as seaweed.

While the invention has mainly been described with reference to certain embodiments, it will be appreciated that various modifications may be made within the scope of the present invention. For example, while embodiments have mainly been described with reference to the detection of salmon lice larvae, it will be appreciated that these embodiments may also be used to detect other types of zooplankton, e.g. skottelus larvae and/or the like. For example, suitable machinelearning algorithms may be trained to detect and/or classify various types of zooplankton. Similarly, the spectral composition of the emitted light may be selected to provide a particularly efficient attraction of the type(s) of zooplankton to be detected. While the illustrated embodiments include a single camera as imaging device, other embodiments may include more than one camera. For example, several microscopy cameras may be imaging the focal plane to increase overall sensitivity. To this end, the cameras may be mounted in a ring formation around the center as a supplement to a central camera. Configurations with 1,2,3, 4 or even more cameras imaging the focal plane at different locations of the surface of the transparent member may e.g. be used.

Claims

1. An apparatus for detecting freely swimming zooplankton in a body of water, the apparatus comprising one or more light sources for emitting visible light and the apparatus being configured to direct a first light portion of the emitted light into the body of water to attract the freely swimming zooplankton towards a detection zone and to direct a second light portion of the emitted light onto the detection zone for illuminating zooplankton within the detection zone, the apparatus further comprising an imaging device configured to capture one or more images of one or more zooplankton present within the detection zone.

2. The apparatus according to claim 1, further comprising an image processing unit configured to process the captured one or more images to recognize one or more zooplankton in the one or more captured images.

3. The apparatus according to any one of the preceding claims, comprising a submersible housing configured to accommodate the one or more light sources and the imaging device and to be submersed into the body of water.

4. The apparatus according to any one of the preceding claims, comprising a communications interface for communicating the captured images and/or the image processing results to an external data processing system and/or to a control system for controlling one or more parasite countermeasures.

5. The apparatus according to any one of the preceding claims, further configured to emit an output light beam via a light-emitting surface area of the submersible housing, wherein a first beam portion of the emitted light beam is emitted as an attraction beam into the body of water for attracting the zooplankton towards the detection zone, and wherein a second beam portion of the emitted light beam is directed towards the detection zone.

6. The apparatus according to claim 5, comprising a reflector, in particular a passive reflector attached to the outside of the submersible housing, the reflector being configured to receive the second beam portion of the emitted light beam and to redirect the second beam portion towards the detection zone.

7. The apparatus according to claim 5 or 6, comprising a transparent surface, wherein a first surface portion of the transparent surface defines the light-emitting surface and a second surface portion of the transparent surface defines the detection zone.

8. The apparatus according to claim 7, wherein the imaging device is configured to have a focal plane at or directly adjacent to the second surface portion of the transparent surface.

9. The apparatus according to any one claims 7 through 8, wherein the first surface portion surrounds the second surface portion, in particular as an annular first surface portion.

10. An apparatus for detecting freely swimming zooplankton in a body of water, the apparatus comprising:

- a housing including a transparent member defining a transparent surface,

- one or more light sources accommodated within the housing for emitting visible light and configured to direct light via a light-emitting portion of the transparent surface into the body of water,

- a reflector arranged outside the housing and configured to receive the light emitted by the one or more light sources via the light-emitting surface portion to redirect the received light onto a detection zone defined by a detection portion of the transparent surface,

11. A process for reducing infestation of sea animals, in particular of fish, such as salmon, more particularly of fish in fish farms, by ectoparasites, in particular salmon lice and/or skottelus, the process comprising: detecting present infestation or a likelihood of imminent infestation of a body of water with ectoparasite larvae, preferably by an embodiment of the apparatus disclosed herein, - controlling one or more ectoparasite countermeasures responsive to a detected infestation or a detected likelihood of imminent infestation.